The behavior of matter in electrical or magnetic field, especially nonuniform fields, is of interest to scientists of various branches: Physics, chemistry, engineering, or life sciences. To chemists and physicists, it's a science of many and varied phenomena. To engineers, it's a source of new and useful techniques for separating, levitating, and rotating materials or improving material behavior.
In recent decades, Dielectrophoresis has become a fairly well known phenomenon in which a spatially nonuniform electric field exerts a net force on the field-induced dipole of a particle. Particles with higher polarizability than the surrounding medium experience positive dielectrophoresis and they move toward regions of highest electric field concentration. Particles less polarizable than the surrounding medium experience negative dielectrophoresis, and move towards regions of low electric field concentration. The force depends on the induced dipole and the electric field gradient, not on the particle's charge. Thus, dielectrophoresis has been used to precipitate DNA and proteins, to manipulate viruses (100 nm diameter), and to manipulate and separate cells and subcellular components such as microtubules.
Dielectrophoretic levitation fulfills a somewhat specialized need among the scientific and technical applications for dielectrophoresis. Two types of levitation, passive and feedback-controlled may be used to levitate particles exhibiting, respectively, negative and positive DEP behavior.
DEP is technologically important in its own right, as evidenced by the number of applications in such scientific and technical fields as biophysics, bioengineering, and mineral separation. As an example, which is important in cancer treatment, is cell fusion, as discussed by P. T. Gaynor, and P. S. Bodger in “Electrofusion processes: theoretical evaluation of high electric field effects on cellular transmembrane potentials”, IEE Proceedings-Science, Measurement and Technology, vol. 142, no. 2, pp. 176-182, 1995. In this process, the nonuniform electric field collects some fraction of these cells on electrode surfaces where cells of the two types inevitably encounter each other and form chains. A serious of short DC pulse is then applied to the electrodes. The strong DC field disturbs the membranes in the region of contact between cells and initiates their merge or fusion. A potential application of this technique is the production of antibodies useful in cancer research and treatment.
Lab-on-a-chip based on DEP phenomenon has become one of the hottest areas of research recently. It has many applications in the biological, pharmaceutical, medical, and environmental fields. These applications are characterized by complex experimental protocols, which need both microorganism detection and manipulation. Hence, lab-on-a-chip technology needs to integrate functions such as: actuation, sensing, and processing to increase their effectiveness. On the other hand, lab-on-a-chip technology holds the promise of cheaper, better and faster biological analysis. However, to date there is still an unmet need for lab-on-a-chip technology to effectively deal with the biological systems at the cell level.
Recently, two different lab-on-a-chip approaches have been proposed by G. Medoro, N. Manaresi, M. Tartagni, and R. Guerrieri, in “CMOS-only Sensors and Manipulation for microorganisms”, Proc. IEDM, pp. 415-418, 2000 and by N. Manaresi, A. Romani, G. Medoro, L. Altomare, A. Leonardi, M. Tartagni, and R. Guerrieri in “A CMOC Chip for Individual Manipulation and Detection”, IEEE International Solid-State Circuits Conference, ISSCC 03, pp. 486-488. 2003. The first, which was proposed in 2002, is the first lab-on-a-chip approach for electronic manipulation and detection of microorganisms. The proposed approach combines dielectrophoresis with impedance measurements to trap and move particles while monitoring their location and quantity in the device. The prototype has been realized using standard printed circuit board (PCB) technology. The sensing part in this approach can be performed by any electrode by switching from the electrical stimulus to a transimpedance amplifier, while all the other electrodes are connected to ground. The second lab-on-a-chip, which was proposed in 2003, is a microsystem for cell manipulation and detection based on standard 0.35 μm CMOS technology. This lab-on-a-chip microsystem comprises two main units: the actuation unit, and the sensing unit. The chip surface implements a 2D array of microsites, each comprising superficial electrodes and embedded photodiode sensors and logic. The actuation part is based on the DEP technique. The sensing part depends on the fact that particles in the sample can be detected by the changes in optical radiation impinging on the photodiode associated with each micro-site. During the sensing, the actuation voltages are halted, to avoid coupling with the pixel readout. However, due to inertia, the cells keep their position in the liquid.
The disadvantage of these lab-on-a-chip microsystems, can be summarized as follows:                Based on these two systems, we can detect the position of the levitated cells. However, we cannot sense the actual intensity of the nonuniform electric field that produces the DEP force.        The measurements here are indirect. In other words, there is no “real-time” detection of the cell response under the effect of the nonuniform electric field, as the actuation part is halted while the sensing part is activated.        The sensing part in these two microsystems depends on the inertia of the levitated cells. In other words, this sensing approach depends on an external factor, which is the inertia of the levitated cells. Thus, only cells with higher inertia can be sensed and detected by using these two microsystems.        
What is needed is a lab-on-a-chip that can be used for direct measurements, where the variations in the electric field can be sensed and the cell can be characterized while the actuation part is still active.